Feller-bunchers are used to fell, or cut down, a number of trees and then to bunch the trees together for transport. The bunched trees are then transported to a storage site for further processing. Typical feller-bunchers are articulated machines that include a front frame and a rear frame that are pivotally coupled by a hitch. The hitch allows the front frame to pivot relative to the rear frame so as to steer the feller-buncher. The front frame includes a front axle having a pair of wheels mounted thereon, and the rear frame includes a rear axle also having a pair of wheels mounted thereon.
An engine mounted on the rear frame supplies mechanical power directly to a drop box. The drop box splits the power output from the engine between the front axle and the rear axle. In particular, a portion of the power from the drop box is supplied directly to the rear axle, and the rest of the power is supplied to the front axle via a drive shaft and universal joint that passes from the rear frame to the front frame at the hitch. A disadvantage to this type of mechanical drive arrangement is that the drive shaft and the universal joint are subjected to high stresses as the front frame is pivoted relative to the rear frame. Thus, the drive shaft and universal joint require regular maintenance to prevent failure. Maintaining or replacing the drive shaft and universal joint is both expensive and time consuming.
The engine also powers a hydraulic system that supplies pressurized hydraulic fluid to several non-drive systems. Generally, the feller-buncher includes a hydraulically powered rotating saw which is used to cut down the trees during a felling operation. In addition, the feller-buncher has a number of hydraulically powered work arms which are used to secure the felled trees to the feller-buncher after the trees have been cut down. Moreover, a number of hydraulic powered cylinders are used to raise, lower, and tilt the rotating saw and work arms. The hydraulic system used to power the rotating saw, the work arms, and the cylinders is separate from the mechanical drive system used to move the work machine from location to location. A disadvantage of the separate systems is that power cannot be transferred between the mechanical drive system and the hydraulic system.
A particular disadvantage of having separate systems is that is it difficult to coordinate the power requirements of the rotating saw and the drive system during a felling operation. The saw must be maintained at a certain minimum rotational speed to cut through the trunks of trees. A problem arises because the power required to cut through the trees increases as the speed of the feller-buncher increases. Thus, as more power is applied to the drive system, the load on the rotating saw increases requiring that additional power be supplied to the rotating saw to maintain the minimum speed. Thus, the total power required to power both the rotating saw and the drive system increases dramatically as the speed of the feller-buncher is increased during a felling operation. However, if the rotating saw and the drive motors shared a common power source, then the increased power demand of the rotating saw would draw power away from the drive system so as to reduce the load on the rotating saw and reduce the total power required to power both the rotating saw and the drive system.
What is needed therefore is an apparatus and method for felling and bunching trees which allows power to be advantageously distributed between the rotating saw and the drive motors which overcome one or more of the above-mentioned drawbacks.